Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import Kevin Lo's port of urtwn(4) from OpenBSD. urtwn(4) is a driver for the
Realtek RTL8188CU/RTL8192CU USB IEEE 802.11b/g/n wireless cards.
This driver requires microcode which is available in FreeBSD ports:
net/urtwn-firmware-kmod.

Hiren ported the urtwn(4) man page from OpenBSD and Glen just commited a port
for the firmware.

TODO:
- 802.11n support
- Stability fixes - the driver can sustain lots of traffic but has trouble
coping with simultaneous iperf sessions.
- fix debugging

MFC after: 2 months
Tested by: kevlo, hiren, gjb